In electrostatic spray coating systems of the type to which this invention relates, coating particles are emitted from a spray device, often called a "gun", toward an object to be coated. The coating particles may be in the form of powder transported to the spray device in a fluid stream such as air, or in the form of liquid such as paint, varnish, lacquer, or the like which has been atomized by the spray device utilizing conventional air atomization, hydraulic atomization ("airless"), and/or rotary atomization principles. Associated with the spray device are one or more electrodes which cause the particles emitted by the spray device to carry an electrostatic charge such that when the charged particles are propelled by the spray device toward an article to be coated, which is maintained at an electrostatic potential different than that of the charged coating particles, the coating particles will be deposited on the article with improved efficiency, coverage, and the like. Depending upon the particular construction of the spray device and its associated electrode(s), the electrical charge transfer mechanism may involve contact charging, corona charging, inductive charging, and/or ionization, etc. in accordance with charging principles which are well known in the electrostatic coating field.
Also associated with the spray device is a high voltage electrostatic supply for providing electrostatic potentials of approximately 50 KV or more to the charging electrode. The high voltage electrostatic supply may be remotely located with respect to the spray device, in which event an electrical cable insulated for high voltage is connected between the spray device and the remote power supply. Illustrative electrostatic liquid spray coating systems of this type are disclosed in Juvinall U.S. Pat. No. 3,367,578 (rotary atomization), Hastings U.S. Pat. No. 4,335,851 (air atomization), and Wilhelm et al U.S. Pat. No. 3,870,233 and Hastings et al U.S. Pat. No. 4,355,764 (hydraulic atomization). A powder spray device supplied from a remote high voltage supply is shown in Duncan et al U.S. Pat. No. 3,746,254. In other known electrostatic spray coating systems, the high voltage electrostatic supply is mounted to and/or incorporated in the spray device, in which case electrical energy is transmitted to the spray device from a remote low voltage source via an electrical cable which need only be insulated for safe operation at low voltage. Illustrative of systems of this latter type are those disclosed in Senay U.S. Pat. No. 3,731,145, Buschor U.S. Pat. No. 3,608,823, Skidmore U.S. Pat. No. 3,599,038, Huber U.S. Pat. No. 4,323,947, and Bentley et al U.S. Pat. No. 4,331,298.
In electrostatic spray coating systems electrical energy is capacitively stored in the electrical path which supplies charging potential to the electrode. Included in this charge-conducting path are components of the high voltage electrostatic supply, interconnecting high voltage cables, and electrical switches, contacts, conductors, and the like. In addition, electrical energy is capacitively stored in the spray device itself as a consequence of the presence of structural elements of an electrically conductive nature which function in much the same manner as plates of a capacitor. The electrical energy stored in capacitive form is proportional to the quantity 1/2 CV.sup.2, where C is capacitance and V is voltage. Should the capacitively stored energy be rapidly discharged, such as, if the electrode is inadvertently electrically grounded or brought in close proximity to an electrically grounded object, a spark can result having sufficient energy to cause ignition in the environment surrounding the spray device which is often explosive due to the presence of volatile coating solvents and/or combustible concentrations of coating powder. Additionally, inadvertent discharge of electrically stored energy can create shock hazards to personnel who come in contact with the charging electrode.
To reduce the rate of discharge of capacitively stored energy in the foregoing situations to safe limits, it has been the practice to connect one or more discrete resistors in the high voltage path which interconnects the charging electrode and the high voltage electrostatic supply. Typically, there is at least one rather large resistor (for example, 75M ohms), and in some cases also a second resistor of lesser value (10M-20M ohms), incorporated in the high voltage path in the spray device or gun upstream of the electrode, with the lesser value resistor preferably being connected directly to the electrode. Illustrative of patents disclosing one or more gun-mounted resistors are Kennon U.S. Pat. No. 4,182,490, and Hastings U.S. Pat. No. 4,335,851, which each disclose a relatively small and a relatively large resistor incorporated in the gun in the electrical path between the electrode and the high voltage cable which connects the spray gun to a remote high voltage electrostatic supply. Illustrative of a single resistor in a gun in the electrical path between the electrode and a high voltage electrostatic supply, also located in the gun, is Skidmore U.S. Pat. No. 3,599,038. Electrostatic coating systems of the rotary atomization type also incorporate discrete resistors in the spray device.
In addition, and in those electrostatic spray coating systems utilizing remotely located high voltage electrostatic supplies, a plurality of discrete resistors are serially connected in the high voltage cable interconnecting the spray gun and the remote high voltage electrostatic supply. Typically, the total resistance of the plural series-connected discrete resistors of the high voltage cable is on the order of approximately two hundred million (200M) ohms. Accordingly, if a cable having a length of eight meters is provided with discrete resistors every one meter of length, each cable resistor will have a value of approximately 25M ohms. Illustrative of one form of high voltage cable incorporating a plurality of series-connected discrete resistors is the cable disclosed in Nord U.S. Pat. No. 3,348,186.
The utilization of discrete resistors, particularly in high voltage cables, has a number of very serious shortcomings. For example, an important disadvantage involves the unreliability, both electrically and mechanically, of discrete resistor high voltage cables, which leads to unpredictable and premature failure. There are a number of causes of this unreliability, including heat dissipation from the resistors which can melt the polyethylene insulation which has a melting point of 200.degree. F., as well as degrade the resistor which also occurs at temperatures of 200.degree. F. or less. Additionally, discrete resistor high voltage cables are not resistant to solvent attack, causing premature failure, and are relatively stiff and bulky, leading to operator fatigue when used with spray devices of the hand-held or manual type.
Another disadvantage of discrete resistor cables is high initial cost due to the relatively high cost of high voltage resistors and the relatively complex assembly process required to electrically and structurally interconnect the series-connected high voltage resistors in the cable. In terms of assembly, the assembly process in one form includes, among other steps, placement of the axial leads of adjacent resistors into conductive vinyl tubes which are used to both physically space and electrically connect adjacent resistors, which is a rather time consuming operation. As for cost, high voltage resistors are themselves quite expensive. The utilization of conductive vinyl tubes into which the resistor leads are inserted are undesirable for a further reason, namely, they cooperate to form a coaxial capacitor giving rise to a still further source of unwanted capacitive electrical energy storage.
Another disadvantage of discrete resistor cables is that while operative in the range of 50 KV-125 KV, they are generally inoperative, at least for extended periods of time, at voltages of 150 KV or more.
High voltage resistors incorporated in the gun, while not as troublesome as discrete resistor cables, nevertheless suffer from a number of the same disadvantages, such as, relatively high cost, inadequate resistance to solvent attack, premature failure, and the like.
In an effort to overcome the problems inherent in discrete resistor high voltage cables, it has been proposed to utilize a high voltage cable having a core fabricated of electrically conductive particles, such as, carbon or graphite granules, distributed within or coated upon a nonconductive material, such as, synthetic or natural rubber. An arrangement of this type is proposed in Point U.S. Pat. No. 3,167,255. The difficulty with this proposal is that the conductivity of the cable core is dependent upon, among other things, surface contact between the conductive particles in the nonconductive matrix, which in turn depends upon the shape and size of the particles as well as the degree to which the particles are uniformly distributed throughout the matrix. Since these variables are extremely difficult to control, it has been found to be virtually impossible to control the resistivity of the cable core within desired limits. Additionally, as the cable is flexed, the conductive particles physically move relative to each other, adversely affecting the conductivity provided by the surface contact between adjacent conductive particles.
A further disadvantage is that the resistivity of the cable core is extremely dependent upon the percentage content of the conductive particles in the nonconductive matrix, with very slight increases in percentage content of conductive particles giving rise to dramatic reductions in resistivity. Since it is virtually impossible to control the percentage content of the conductive particles with the precision required, the resistivity of the cable core is highly erratic from cable to cable and/or from one section to another within the same cable.
Proposals for conductive particle-type resistive elements, although not for use in high voltage cables for electrostatic spray coating systems, are contained in Asakawa U.S. Pat. No. 2,861,163, Weckstein U.S. Pat. No. 3,859,506, French Pat. No. 983753. Asakawa proposes a heating element having conductive carbon black particles distributed in nonconductive material, such as, paraffin, polyethylene, etc. Weckstein proposes a heating cable having several layers of different type material, one layer of which includes "high resistance conductive" yarn in the form of "electrically conductive strands of fiberglass or quartz subjected to milimicron-size particles of a highly conductive material in a colloidal suspension". Illustrative of colloidal particles which are proposed are those of "graphite, silicon carbon, and other semiconducting materials". The French patent also appears to refer to the use of silicon carbide powder as an impregnating material in an otherwise nonconducting fiber core of an ignition cable. For the reasons noted in connection with Point U.S. Pat. No. 3,167,255, namely, reliance upon conductive particulate material in an insulative matrix, the proposals of the foregoing patents suffer the noted disadvantages of resistance change with flexion, inability to control resistivity, etc. which are inherent in "conductive particle" type cables.
Holtzberg U.S. Pat. No. 4,369,423 proposes an electrically conductive automotive ignition cable which has a core comprising a plurality of mechanically and electrically continuous filaments of graphitized polyacrylonitrile. The Holtzberg graphitized polyacrylonitrile filament automotive ignition cable has a resistance of approximately 200 ohms per lineal meter. While a resistance per lineal meter of this magnitude is presumably acceptable in the Holtzberg application where the objective is to provide reduced RF disturbance and resistance in an automotive ignition cable, it is totally inoperative for use as a high voltage cable in an electrostatic spray coating system where a resistance per lineal meter of approximately 30M (30 .times.10.sup.6) ohms is typically necessary. As used herein, the numeric abbreviation M is defined to equal 10.sup.6.